As the nanoelectronics industry pushes towards feature sizes of 22 nm and smaller, conventional single-exposure refractive lithography systems used to print circuit patterns onto computer chips will no longer be feasible. Extreme ultraviolet (EUV) lithography, utilizing reflective optics and 13-nm-wavelength light to print chips, is the leading candidate to meet the industry’s future needs. Despite strong progress in EUV lithography over the past decade, significant challenges remain, including defect-free mask fabrication (see Science Highlight Investigating Extreme Ultraviolet Lithography Mask Defects), and the development of ultrahigh-resolution photoresist—a light-sensitive material used to form a patterned coating—that simultaneously supports low line-edge roughness (LER), high sensitivity, and sub-22-nm resolution. Using the SEMATECH Berkeley Microfield Exposure Tool (MET) at ALS Beamline 12.0.1.3, advanced EUV photoresist research can be performed while high-power stand-alone light sources are still being developed. High-quality 16-nm lines and spaces have been printed using the MET, representing the highest resolution ever achieved from a single-exposure projection optical lithography tool.

Staying Ahead of the Game

Nanoelectronics technology will allow the semiconductor industry to continue on its historic path of ever-increasing density and improving functionality: increased memory capacities, improved computing speeds, reduced costs, and new functionality in devices. In accordance with Moore’s Law, the number of transistors on a computer’s central processing unit doubles every two years. To put this continued progress into perspective, the four gigabytes of memory in a $70 iPod shuffle would have cost $4B in 1970.

EUV lithography will allow this rate of improvement to continue, with high-volume production for patterning of 22 nm half-pitch features expected in 2013. In light of the ongoing commercialization of nanoelectronics technology in general, and EUV lithography specifically, advanced research in materials enabling the extension of this technology to below 16 nm is required today. To address this need, researchers have been using the MET at ALS Beamline 12.0.1.3 since 2003, and an even higher-resolution capability is being developed at the ALS in partnership with SEMATECH, allowing materials science researchers to keep the pace of resist development well ahead of production capabilities.

The SEMATECH Berkeley MET at ALS Beamline 12.0.1.3 (from right to left): A rack of custom-made control electronics which allow researchers to meticulously manipulate the tool. The MET is located within a thermally controlled enclosure (shiny metal paneling) that can maintain temperatures to 1/100th of a degree centigrade inside of the projection optics chamber. A researcher inspects the photoresist on exposed film in the post-exposure clean room following development.

Development of EUV photoresist is particularly challenging because it relies on the availability of ultrahigh resolution lithography tools well in advance of when they would be required for high-volume semiconductor manufacturing. Tools such as the MET allow researchers to study nanoscale patterning at EUV wavelengths, and so play a crucial role in the advanced materials development required to achieve high-volume fabrication of nanoscale electronics. The relative simplicity of these microcfield tools enables them to provide higher-resolution capabilities than current full-production-scale tools. For example, preproduction tools just now being delivered still significantly underperform in terms of resolution when compared to the MET, which came online late in 2003.

The most unique attribute of the SEMATECH Berkeley MET is its use of a custom-coherence illuminator made possible by its implementation on a synchrotron beamline. Using conventional illumination, the resolution limit of the 0.3-NA optic MET is approximately 25 nm; however, with custom-coherence the system can achieve the wavelength-induced resolution limit of 12 nm. Moreover, the high spectral purity of the ALS light minimizes image degradation, which can be caused by longer ultraviolet wavelengths present in current plasma-based EUV light sources under development.

Exposures of 45-nm lines and spaces using both conventional illumination and the pseudo phase shift mask method, which results in twice as many 22.5-nm lines and spaces.

Another practical problem for EUV photoresist testing at these small dimensions is that masks supporting these resolutions must also be available. To decouple resist research from mask development, a process unique to the ALS implementation of the MET called pseudo phase shift mask is used, allowing conventional binary amplitude masks to act as phase shift masks, causing line space features to print at one half the size they are on the mask. Using this method, high-quality 16-nm line space printing with 2-nm LER has been achieved in an experimental photoresist. Although still lacking in sensitivity and edge roughness, these results represent a significant breakthrough in single-exposure projection printing resolution.

Using an experimental photoresist provided by Inpria Corporation, 16-nm lines and spaces were printed on the MET using pseudo phase shift mask mode.

These results were obtained in collaboration with Inpria Corporation, who has developed a photosensitive spin-on inorganic ultrathin imaging film (resist). The inorganic nature of the film enables high EUV absorption in the ultrathin film, minimizing stochastic effects. Additionally, the film does not rely on thermal chemical amplification, thereby reducing diffusion and image blur during post-exposure processing. The downside of the lack of chemical amplification is decreased sensitivity. The sensitivity of the resist shown in the image above, for example, was 70 mJ/cm2, which is 4–7 times slower than industry requirements.